Session Notes

Session: Field Experience

Broken glass appeared to be the results of thermal expansion issues associated with
the racking system.

The solder bond failures reported in "Reliability Evaluation of PV Power Plants: Input
data for Warranty, Bankability and Energy Estimation Models" were judged to be more
related to workmanship than design, but the detailed cause has not been determined.

The bypass diode failures reported in "Reliability Evaluation of PV Power Plants:
Input data for Warranty, Bankability and Energy Estimation Models" resulted in open-circuited
bypass diodes. Almost all of these were found in a system that had shading.

Session: Potential Induced Degradation

The comparison of damp heat and foil testing presented in "Testing Modules for Potential-Induced
Degradation — A Status Update of IEC 62804" would have given a better comparison if
weight had been applied to the foil.

Some attendees raised the question of using a foil test that applies the stress across
the entire module instead of simulating the situation in the field. Others noted that
the foil test does a good job of simulating the leakage when it is raining and that
they like the foil test because it helps to easily identify whether there is a problem.
The current draft of IEC 62804 includes both the damp heat and the foil method.

Volker Naumann reported that after a regenerative bias has been applied (after degradation
induced by application of a bias voltage), they have observed evidence that the sodium
migrated back out of the stacking faults. Annealing can also lead to regeneration,
but takes a longer time.

Session: Physics of Failure

In "Guidelines for Comparing indoor Accelerated Stress Tests to Outdoor Use," it was
assumed that there was a single reaction occurring with a single activation energy,
but in some situations there are multiple processes occurring that have different
activation energies. In some cases, the rate-limiting step may be the diffusion of
the moisture, which gives a different result than modeling the activation energy of
the chemical reaction.

We need to be cognizant of the difference between the application of damp heat and
the photolysis process that occurs in the field. To be quantitative, we need to model
the specific reactions.

Measuring the activation energy may give a clue to the mechanism.

When looking at the effects of moisture ingress, one should include thermal cycling.
Inclusion of thermal variation may open paths for moisture ingress that one would
not see in an isothermal damp-heat test.

Session: Inverter Reliability

A separate study showed that 1000 cycles corresponded to 10 y in the field, but the
details of the temperature fluctuations during those 10 y are unknown.

When comparing the effect of temperature fluctuations in the field with thermal cycling
in a chamber, the order of the fluctuations may change the damage done by a specific
change in temperature. There's a hysteresis effect.

The temperatures experienced for microinverters are higher than ambient. Typically,
SolarBridge keeps the inverters 3-4 inches off of the roof, which keeps the temperature
in a manageable range.

During testing, Advanced Energy installs as many as 200 thermocouples to monitor every
IGBT, every PC board, and many other places.

To enable the writing of good standards, we need the manufacturers to communicate
about the failures modes that are being seen.

There is talk of people disconnecting from the grid, but battery storage is still
expensive and unreliable, so its time has not yet come.

There is a trend toward undersizing the inverter, then clipping the power (by operating
the modules in forward bias) when the inverter rating is exceeded. Advanced Energy
reports that some of their 1 MW inverters may clip as much as 8 hours in a day. The
design intent is to have a steep power-up curve.

The operating voltage and power point (clipping) need to be considered when testing
for degradation and failure mechanisms.

Testing the firm ware is essential. Advanced Energy uses a hardware emulator to test
the firmware in the laboratory and also tests firmware on site in beta testing. Much
of this testing is directed by UL standards.

Customers want longer reliability from inverters. However, this message isn't driving
the standards development — customers need to participate in standards writing to
demand that testing is done to give the confidence that customers look for.

Session: Quality Management

Hand-soldering can yield superb results, but movement toward automation may help to
provide better consistency when there is a diverse work force.